Lignin-polycaprolactone (PCL) copolymers were prepared using a highly efficient and nontoxic zinc catalyst. The reaction kinetics were investigated and differ from those usually observed in ring-opening polymerization. Depending on the chain length of the copolymer, an acceleration of the reaction was found. These new findings help to understand the mechanisms of lignin functionalization and contribute to a more sustainable plastics industry.

Abstract

With 300 billion tons available in the biosphere, lignin is the second most abundant biopolymer on Earth. However, less than two percent is used for value-added applications. One potential application is the use of lignin as a building block for thermoplastics. The majority of plastics today are made from fossil-based feedstocks. Therefore, the use of lignin can counteract the increasingly scarce petroleum resources. A highly useful approach is the copolymerization with cyclic lactones such as caprolactone (CL) via ring-opening polymerization (ROP). The synthesis of lignin-polycaprolactone (PCL) copolymers can help to combine the beneficial properties of PCL and lignin to create potential new applications. In this work, lignin-PCL copolymers are synthesized in a sustainable approach using the nontoxic, highly active, and robust zinc-based guanidine catalyst [Zn{(R,R)-DMEG₂(1,2)ch}₂]OTf₂ · THF. Analyzing the reaction kinetics, it was found that the pseudo-first order reaction kinetics do not proceed with a uniform rate constant over the entire reaction. An acceleration occurs after the initial formation of PCL chains at the lignin core, with reaction rates depending on both the catalyst and lignin content. These new findings contribute to the mechanistic understanding behind lignin functionalization, highlighting the potential of such bio-based copolymers for a sustainable plastic use.